Remote sensing using light pulses emitted for example by lasers and retroreflected by distant objects is sometimes also referred to as LIDAR (light detection and ranging). A LIDAR receiver, hereinafter also referred to as a front-end receiver, includes an optical receiver having a photodiode (PD) or an avalanche photodiode (APD) as a receiving element, and a transimpedance amplifier (TIA), for example a shunt-feedback amplifier, which converts the photocurrent from the receiving photodiode into a voltage.
In the following, the terms photodiode (PD) and avalanche photodiode (APD) will be used interchangeably, unless otherwise stated. A photodiode is typically a p-n junction or PIN structure. When a photon of sufficient energy strikes the diode, it creates an electron-hole pair. This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up. When used in photoconductive mode, the diode is often reverse-biased (with the cathode driven positive with respect to the anode). This reduces the response time because the additional reverse bias increases the width of the depletion layer, which decreases the junction's capacitance. The reverse bias also increases the dark current without much change in the photocurrent. For a given spectral distribution, the photocurrent is linearly proportional to the illuminance (and to the irradiance).
APDs can be thought of as photodetectors that provide a built-in first stage of gain through avalanche multiplication. From a functional standpoint, they can be regarded as the semiconductor equivalent to photomultipliers. Due to their high sensitivity, a typical application for APD's is in laser rangefinders and long range fiber-optic telecommunication.
In some applications, an APD generates a current pulse proportional to the received electromagnetic power and a TIA converts the current pulse into a voltage pulse and also provides a high gain in order to detect weaker signals from distant objects. For closer objects, the magnitude of the current pulse at the input of the TIA can reach the limits of linear operation of the TIA. In such cases, the TIA becomes saturated. In shunt-feedback amplifier topology, saturation causes the output voltage pulse to widen by a certain amount, which is referred to as pulse-width distortion. Upon overloading, such transimpedance amplifiers have very long relaxation times until again the TIA can return to linear operation.